Regime bombardment on northwestern Syria killed at least seven civilians including children on Tuesday, a monitor said, a day after dozens were killed in raids targeting the opposition bastion.

The Syrian regime and its Russian ally have stepped up their deadly bombardment of the Idlib region and adjacent areas in Aleppo and Hama provinces since late April.

Three children were among seven civilians killed in Tuesday's air strikes and artillery fire in Aleppo and Idlib provinces, the Britain-based Syrian Observatory for Human Rights said.

It came a day after regime and Russian air strikes killed 50 people in northwestern Syria.

Thirty-eight of them perished in raids on a vegetable market and surrounding areas in the town of Maaret al-Numan in Idlib province.

The jihadist-run Idlib region is home to some three million people and is supposed to be protected by a months-old international truce deal.

The spike in violence since late April has killed more than 700 civilians, including more than 170 children, according to the Observatory.

More than 330,000 people have fled violence in the area over the past three months, according to the United Nations.

Mark Cutts, UN deputy regional humanitarian coordinator for the Syria crisis, described a worsening "nightmare" unfolding in Idlib.

"Today saw one of the deadliest attacks on civilian areas that we have seen since the upsurge in fighting began almost three months ago," he said in a Monday statement.

Syria's war has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-government protests.